Most commercial low density polyethylenes are polymerized in heavy walled autoclaves or tubular reactors at pressures as high as 50,000 psi and temperatures up to 300.degree. C. The molecular structure of high pressure low density polyethylene is highly complex. The permutations in the arrangement of its simple building blocks are essentially infinite. High pressure resins are characterized by an intricate long chain branched molecular architecture. These long chain branches have a dramatic effect on the melt rheology of the resins. High pressure low density polyethylene resins also possess a spectrum of short chain branches generally 1 to 6 carbon atoms in length which control resin crystallinity (density). The frequency distribution of these short chain branches is such that, on the average, most chains possess the same average number of branches. The short chain branching distribution characterizing high pressure low density polyethylene can be considered narrow.
The term "linear" is defined as identifying a polymer chain which is predominantly free of long chain branching By "predominantly free of long chain branching" is meant less than 0.5 branches/per 1000 carbon atoms in the polyethylene molecule. "Long chain branching" characterizes branching within polymeric structures which exceeds short branch lengths of pendant groups derived from individual alpha-olefin comonomers. A long chain branch of polyethylene should have at least a sufficient number of carbon atoms to provide significant modifications in rheological behavior, such as caused by chain entanglement. The minimum number of carbon atoms is usually greater than about 100. Short chain branching introduced through comonomer polymerization provides branch lengths of usually less than about 10 carbon atoms per branch. Non-crosslinked linear low density polyethylene (LLDPE) possesses little, if any, long chain branching such that the only branching to speak of is short chain branching, with such branch length controlled by the pendant chain length of the comonomeric alpha-olefins provided.
The term "narrow molecular weight distribution" as used herein refers to the ratio of weight average molecular weight to number average molecular weight. This ratio can be between 1 and about 10, preferably between about 2 to about 6.5, and most preferably between about 3 to about 5. The lower limit of this ratio is defined by the theoretical limit since number average molecular weight cannot exceed weight average molecular weight by definition.
Low density polyethylene can exhibit a multitude of properties. It is flexible and has a good balance of mechanical properties such as tensile strength, impact resistance, burst strength, and tear strength. In addition, it retains its strength down to relatively low temperatures. Certain resins do not embrittle at temperatures as low as -70.degree. C. Low density polyethylene has good chemical resistance, and it is relatively inert to acids, alkalis, and inorganic solutions. It is, however, sensitive to hydrocarbons, halogenated hydrocarbons, and to oils and greases. Low density polyethylene has excellent dielectric strength.
More than 50% of all low density polyethylene is processed into film. This film is primarily utilized in packaging applications such as for meat, produce, frozen food, ice bags, boilable pouches, textile and paper products, rack merchandise, industrial liners, shipping sacks, pallet stretch and shrink wrap. Large quantities of wide heavy gauge film are used in construction and agriculture.
Most low density polyethylene film is produced by the tubular blown film extrusion process. Film products made by this process range in size, from tubes which are about two inches or less in diameter, and which are used as sleeves or pouches, to huge bubbles that provide a lay flat of up to about twenty feet in width, and which, when slit along an edge and opened up, will measure up to about forty feet in width.
Polyethylene can also be produced at low to medium pressures by homopolymerizing ethylene or copolymerizing ethylene with various alpha-olefins using heterogeneous catalysts based on transition metal compounds of variable valence. These resins generally possess little, if any, long chain branching and the only branching to speak of is short chain branching. Branch length is controlled by comonomer type. Branch frequency is controlled by the concentration of comonomer(s) used during copolymerization. Branch frequency distribution is influenced by the nature of the transition metal catalyst used during the copolymerization process. The short chain branching distribution characterizing transition metal catalyzed low density polyethylene can be very broad.
Linear low density polyethylene can also be produced by high pressure techniques as is known in the prior art.
U.S. Pat. No. 4,302,566 in the names of F. J. Karol et al and entitled Preparation of Ethylene Copolymers in Fluid Bed Reactor, discloses that ethylene copolymers, having a density of 0.91 to 0.96, a melt flow ratio of greater than or equal to 22 to less than or equal to 32 and a relatively low residual catalyst content can be produced in granular form, at relatively high productivities if the monomer(s) are copolymerized in a gas phase process with a specific high activity Mg-Ti containing complex catalyst which is blended with an inert carrier material.
U.S. Pat. No. 4,302,565 in the names of G. L. Goeke et al and entitled Impregnated Polymerization Catalyst, Process for Preparing, and Use for Ethylene Copolymerization discloses that ethylene copolymers, having a density of 0.91 to 0.96, a melt flow ratio of greater than or equal to 22 to less than or equal to 32 and a relatively low residual catalyst content can be produced in granular form, at relatively high productivities, if the monomer(s) are copolymerized in a gas phase process with a specific high activity Mg-Ti-containing complex catalyst which is impregnated in a porous inert carrier material.
The polymers as produced, for example, by the processes of said patents using the Mg-Ti containing complex catalyst possess a narrow molecular weight distribution, Mw/Mn, of about greater than or equal to 2.7 or less than or equal to 4.1.
The occurrence of melt fracture during extrusion of high molecular weight ethylene polymers having a narrow molecular weight distribution have been discussed extensively in U.S. Pat. Nos. 4,522,776; 4,552,712; and 4,554,120 the contents of which are incorporated herein by reference. Thus, U.S. Pat. No. 4,522,776, issued to A. V. Ramamurthy on Jun. 11, 1985 eliminates melt fracture by using a die having a die land surface fabricated of a material which increases adhesion between the die land surface and the polymer.
U.S. Pat. No. 4,552,712 issued to A. V. Ramamurthy on Nov. 12, 1985 reduces melt fracture by using a die having a die land region fabricated from stainless steel and wherein the length of the die land to the width of the die gap is about 35:1 to about 60:1.
U.S. Pat. No. 4,554,120 issued to A. V. Ramamurthy on Nov. 19, 1985 substantially eliminates surface melt fracture by using a die having a die land region defining opposing surfaces at least one of which is fabricated from an alloy containing 5 to 95 parts by weight zinc and 95 to 5 parts by weight copper.
Films suitable for packaging applications must possess a balance of key properties for broad end use utility and wide commercial acceptance. These properties include film optical quality, for example, haze, gloss, and see through characteristics. Mechanical strength properties such as puncture resistance, tensile strength, impact strength, stiffness, and tear resistance are important. Vapor transmission and gas permeability characteristics are important considerations in perishable goods packaging. Performance in film converting and packaging equipment is influenced by film properties such as coefficient of friction, blocking, heat sealability and flex resistance. Low density polyethylene has a wide range of utility such as in food packaging and non food packaging applications. Bags commonly produced from low density polyethylene include shipping sacks, textile bags, laundry and dry cleaning bags and trash bags. Low density polyethylene film can be used as drum liners for a number of liquid and solid chemicals and as protective wrap inside wooden crates. Low density polyethylene film can be used in a variety of agricultural and horticultural applications such as protecting plants and crops, as mulching, for storing of fruits and vegetables. Additionally, low density polyethylene film can be used in building applications such as a moisture or moisture vapor barrier. Further, low density polyethylene film can be coated and printed for use in newspapers, books, etc.
Possessing a unique combination of the aforedescribed properties, high pressure low density polyethylene is the most important of the thermoplastic packaging films. It accounts for about 50% of the total usage of such films in packaging. Films made from the polymers of the present invention, preferably the ethylene hydrocarbon copolymers, offer an improved combination of end-use properties and are especially suited for many of the applications already served by high pressure low density polyethylene.
An improvement in any one of the properties of a film such as elimination or reduction of surface melt fracture or an improvement in the extrusion characteristics of the resin or an improvement in the film extrusion process itself is of the utmost importance regarding the acceptance of the film as a substitute for high pressure low density polyethylene in many end use applications.